This invention relates generally to devices for the treatment of spinal conditions, and more particularly, to the treatment of various spinal conditions that cause back pain. Even more particularly, this invention relates to devices that may be placed between adjacent spinous processes to treat various spinal conditions. For example, spinal conditions that may be treated with these devices may include spinal stenosis degenerative disc disease (DDD), disc herniations and spinal instability, among others.
The clinical syndrome of neurogenic intermittent claudication due to lumbar spinal stenosis is a frequent source of pain in the lower back and extremities, leading to impaired walking, and causing other forms of disability in the elderly. Although the incidence and prevalence of symptomatic lumbar spinal stenosis have not been established, this condition is the most frequent indication of spinal surgery in patients older than 65 years of age.
Lumbar spinal stenosis is a condition of the spine characterized by a narrowing of the lumbar spinal canal. With spinal stenosis, the spinal canal narrows and pinches the spinal cord and nerves, causing pain in the back and legs. It is estimated that approximately 5 in 10000 people develop lumbar spinal stenosis each year. For patients who seek the aid of a physician for back pain, approximately 12%-15% are diagnosed as having lumbar spinal stenosis.
Common treatments for lumbar spinal stenosis include physical therapy (including changes in posture), medication, and occasionally surgery. Changes in posture and physical therapy may be effective in flexing the spine to decompress and enlarge the space available to the spinal cord and nerves—thus relieving pressure on pinched nerves. Medications such as NSAIDS and other anti-inflammatory medications are often used to alleviate pain, although they are not typically effective at addressing spinal compression, which is the cause of the pain.
Surgical treatments are more aggressive than medication or physical therapy, and in appropriate cases surgery may be the best way to achieve lessening of the symptoms of lumbar spinal stenosis and other spinal conditions. The principal goal of surgery to treat lumbar spinal stenosis is to decompress the central spinal canal and the neural foramina, creating more space and eliminating pressure on the spinal nerve roots. The most common surgery for treatment of lumbar spinal stenosis is direct decompression via a laminectomy and partial facetectomy. In this procedure, the patient is given a general anesthesia and an incision is made in the patient to access the spine. The lamina of one or more vertebrae may be partially or completely removed to create more space for the nerves. The success rate of decompressive laminectomy has been reported to be in excess of 65%. A significant reduction of the symptoms of lumbar spinal stenosis is also achieved in many of these cases.
The failures associated with a decompressive laminectomy may be related to postoperative iatrogenic spinal instability. To limit the effect of iatrogenic instability, fixation and fusion may also be performed in association with the decompression. In such a case, the intervertebral disc may be removed, and the adjacent vertebrae may be fused. A discectomy may also be performed to treat DDD and disc herniations. In such a case, a spinal fusion would be required to treat the resulting vertebral instability. Spinal fusion is also traditionally accepted as the standard surgical treatment for lumbar instability. However, spinal fusion sacrifices normal spinal motion and may result in increased surgical complications. It is also believed that fusion to treat various spinal conditions may increase the biomechanical stresses imposed on the adjacent segments. The resultant altered kinematics at the adjacent segments may lead to accelerated degeneration of these segments.
As an alternative or complement to the surgical treatments described above, an interspinous process device may be implanted between adjacent spinous processes of adjacent vertebrae. The purposes of these devices are to provide stabilization after decompression, to restore foraminal height, and to unload the facet joints. They also allow for the preservation of a range of motion in the adjacent vertebral segments, thus avoiding or limiting possible overloading and early degeneration of the adjacent segments as induced by fusion. The vertebrae may or may not be distracted before the device is implanted therebetween. An example of such a device is the interspinous prosthesis described in U.S. Pat. No. 6,626,944, the entire contents of which are expressly incorporated herein by reference. This device, commercially known as the DIAM® spinal stabilization system, is designed to restabilize the vertebral segments as a result of various surgical procedures or as a treatment of various spinal conditions. It limits extension and may act as a shock absorber, since it provides compressibility between the adjacent vertebrae, to decrease intradiscal pressure and reduce abnormal segmental motion and alignment. This device provides stability in all directions and maintains the desired separation between the vertebral segments all while allowing motion in the treated segment.
Although currently available interspinous process devices typically work for their intended purposes, they could be improved. For example, where the spacer portion of the implant is formed from a hard material to maintain distraction between adjacent vertebrae, point loading of the spinous process can occur due to the high concentration of stresses at the point where the hard material of the spacer contacts the spinous process. This may result in excessive subsidence of the spacer into the spinous process. In addition, if the spinous process is osteoporotic, there is a risk that the spinous process could fracture when the spine is in extension. In addition, because of the human anatomy and the complex biomechanics of the spine, some currently available interspinous process devices may not be easily implantable in certain locations in the spine.
The spine is divided into regions that include the cervical, thoracic, lumbar, and sacrococcygeal regions. The cervical region includes the top seven vertebrae indentified as C1-C7. The thoracic region includes the next twelve vertebrae identified as T1-T12. The lumbar region includes five vertebrae L1-L5. The sacrococcygeal region includes five fused vertebrae comprising the sacrum. These five fused vertebrae are identified as the S1-S5 vertebrae. Four or five rudimentary members form the coccyx.
The sacrum is shaped like an inverted triangle with the base at the top. The sacrum acts as a wedge between the two iliac bones of the pelvis and transmits the axial loading forces of the spine to the pelvis and lower extremities. The sacrum is rotated anteriorly with the superior endplate of the first sacral vertebra angled from about 30 degrees to about 60 degrees in the horizontal plane. The S1 vertebra includes a spinous process aligned along a ridge called the medial sacral crest. However, the spinous process on the S1 vertebrae may not be well defined, or may be non-existent, and therefore may not be adequate for supporting an interspinous process device positioned between the L5 and S1 spinous processes.
Thus, a need exists for an interspinous process device that may be readily positioned between the L5 and S1 spinous processes. Moreover, there is a need to provide an interspinous process device that can provide dynamic stabilization to the instrumented motion segment and not affect adjacent segment kinematics.